The early moderns elevated human reason, downplaying the epistemic role of revelation. Some also questioned the safety of relying on sense experience. In the 17th century, René Descartes proposed a method of systematic doubt to clear away every basis for thought that could be called into question. His aim was to find an a priori basis for it instead. Although we may question the certainty of the foundation that he claimed to find (the famous cogito ergo sum), we can recognize the cogency of his demand for reliable foundations for thinking. Here, we will recapitulate some of the reasons for calling sense experience into question, examine the alleged need for “certain” foundations for thought, and show how that quest for certainty can have radically skeptical results.
Many early modern Western thinkers elevated the epistemic role of reason over revelation. Modern rationalism, a part of the Enlightenment, is identified with such thinkers as Descartes, Leibniz, and Spinoza. The Enlightenment was not “new.” It came from a genuine renaissance and harked back to an ancient Greek epistemic model that the medieval “era of faith” had subordinated.
Some early moderns also questioned the safety of relying on sense experience as a basis for our rational exercises. This kept a good “Christian” mistrust of fleshly things center stage. However, it also made this aspect of the Greek revival more Platonic than Aristotelian. Nevertheless, revelation was not the thing either. But if a reliable basis for thought is not provided by revelation or by sensation, where can it come from? Early modern rationalists, such as Descartes, were foundationalists who believed that there are basic or foundational items of knowledge that we have from which we can reason to the full array of knowledge that we seek.
In the 17th century, René Descartes proposed a method of systematic doubt to clear away every basis for thought (every sort of content source) that could be called into question.
His aim was to find an a priori basis for it instead. What he arrived at is the famous cogito ergo sum (which he wrote in French—je pense, donc je suis). The cogito is supposed to be beyond doubt. Doubt itself is a form of thought, and whatever thinks is. Everything else can be doubted. Sense experience, as we have seen, is notoriously unreliable. Revelation, by the same token, is also unreliable, because what may seem to be a revelation from God might be the result of the interference of an “evil genius.” Even if we agree with Descartes in believing that the occurrence of doubt entails the occurrence of thought and that the occurrence of thought entails the occurrence of a thinker, his claim that “I think” entails “I am” does not necessarily follow. As Bishop George Berkeley later showed, alternatives to the “I” (the substantial ego) are readily available (for example, the mind of God). Although we may question the self-evidence, certainty, or intuitive necessity of the cogito, we can understand Descartes’ mistrust of sensation and recognize the cogency of his demand for an unshakable foundation for thinking.
There is a mathematical model at work here again: Conclusions are to be rationally derived from necessarily true axioms (emulating Euclid’s theorems that were said to be grounded in the necessarily true axioms of geometry). This model is, once again, strongly reminiscent of Plato. The model is put directly in play when Descartes uses Euclidian geometry to show how his ontological proof of the existence of God works (which proof, please note, appeals directly to reason, not to religious experience and/or revelation, however “theological” its topic). The proof of God, if it works, eliminates the possibility that our reasoning is being manipulated by an “evil genius.” This model requires that its axioms (starting points) actually be self-evident, necessary, or logically true. But are statements about matters of fact (for example, the statements of applied geometry) ever selfevident, necessary, or logically true? Or, conversely, do logically true statements (for example, the statements of formal geometry) necessarily have any factual content or application? Isn’t their applicability to the world contingent? Non-Euclidean geometries—in which the interior angles of a triangle don’t add up to 180°—apply very well in an area of intense gravitation where, according to Einstein’s theories of relativity, space itself is distorted or bent, and triangles drawn there are not Euclidian.
Descartes’ rational reconstruction of knowledge is based on what he takes to be a self-evident and necessary truth from which he aims to reconstruct a full understanding of the world around us, but the self-evident truth that he starts with is not necessarily self-evident. The model that he follows is based on a conventional and arbitrary starting point, and its applicability is a contingent matter of fact, not a matter of logical necessity.
[Courtesy: Professor James Hall]